堆積盆シミュレーションによる女川層タイトオイルのポテンシャル評価

抄録

The Miocene Onnagawa Formation, lithologically similar to the Monterey Formation, mainly consists of siliceous rocks, which are known as the main source rock in the Akita area, northern Japan. In the Yurihara-Ayukawa conventional oil and gas field, the Onnagawa Formation forms traps in the depths, vicinity of the opal-CT/quartz phase transition zone. In this area, it is confirmed that opal-CT porcelanites take a role of seals and the migrated hydrocarbons accumulate within quartz porcelanites. However, in the matured basin center area, it is not confirmed that the generated hydrocarbons remain in-situ to form self-source tight oil reservoir, and the saturation has not been tested because of few wells drilled. In this study, we simulated the process of hydrocarbon generation, migration and accumulation using basin modeling technique.

Before applying the simulation, we performed laboratory measurement to analyze kinetics, helium porositypermeability, and mercury injection capillary pressure. Siliceous rocks in the study area can be divided into two major facies, clay-rich porcelanite and clay-poor porcelanite. Based on the laboratory analysis results, we established the rock property models of each facies, and assigned them to the 3D facies model as the input data for basin modeling.

The simulation results demonstrate that the generated hydrocarbons within clay-rich facies could not be expelled because of low permeability, and remained in-situ. As most of TOC values of the Onnagawa Formation are less than

2%, the maximum oil saturation of clay-rich facies is approximately 20% even in the matured area. By contrast, highly permeable clay-poor facies is thought to work as carrier beds. The generated hydrocarbons within the clay-poor facies are thought to be migrated and accumulated in structural and stratigraphic traps within the Onnagawa Formation. The oil saturation is up to 70%, and clay-poor facies has a potential to be a main target as tight oil reservoir.